1. Field of the Invention
This invention relates to a urethane composition comprising a base compound and, as a curing agent, a liquid product of a toluene diisocyanate tar. The invention also relates to a process for producing a liquid product of a toluene diisocyanate tar occurring as a by-product in the preparation of toluene diisocyanate, which comprises dissolving or dispersing the tar in a specific aromatic solvent.
2. Description of the Prior Art
Polyols have long been known as a base compound for obtaining urethane compositions, and in recent years, it has become clear that an isocyanate-reactive tar obtained from a tar occurring as a by-product in the synthesis of phenols through hydroperoxides (to be referred to hereinafter for brevity as "by-product tar") is quite superior as the base compound. Urethane compositions obtained from such base compounds and curing agents composed of isocyanate compounds have found a wide range of applications in, for example, paints, road-paving compositions, adhesives, foamed materials, or flooring materials. Examples of isocyanate compounds include toluene diisocyanate (TDI), xylylene diisocyanate, hexamethylene diisocyanate, diphenylmethane diisocyanate, an adduct of toluene diisocyanate and trimethylol propane, and high-molecular-weight polyisocyanates (prepolymers) obtained from these isocyanates. However, these isocyanate compounds are generally expensive, and especially when it is desired to obtain urethane compositions of excellent strength and chemical resistance, more expensive polyisocyanates are required. Accordingly, the resulting urethane compositions naturally become costly, and isocyanate compounds available at low cost have been greatly desired in order to utilize the superior properties of urethane compositions in an even wider range of applications.
It is known that in the production of toluene diisocyanate, a tar-like substance (to be referred to hereinafter simply as a toluene diisocyanate tar or TDI tar) inevitably occurs as a by-product, and no method is known to utilize this by-product effectively. With increased production of toluene diisocyanate in recent years, several tens of thousands of tons of the TDI tar have been formed annually all over the world, and simply disposed of as industrial wastes. If this by-product could be effectively utilized, a wide range of utility would be found because of the reactivity of the isocyanate group contained in the material, and great economic advantages would accrue in substituting this material for expensive isocyanate compounds.
The properties of the TDI tar vary somewhat according to the difference in the manufacturing conditions employed in producing toluene diisocyanate (TDI). Generally, however, the TDI tar is a black brown solid which contains 10 to 30% by weight of free isocyanate groups (-NCO) and which is brittle at room temperature. When TDI tar is heated to a temperature of above 150.degree. C, usually above 200.degree. C, that is, above the softening point, the tar melts, and changes to a viscous liguid. When the viscous liquid is further heated to higher than 250.degree. to 260.degree. C, the isocyanate groups decompose and gases evolve. Furthermore, when allowed to stand in the air, the TDI tar undergoes changes with the passage of time as a result of the gradual reaction of the isocyanate groups with the moisture in the air.
A great problem in the production of TDI industrially is that no method for effectively utilizing the TDI tar is at present known, and the disposal of the enormous amount of the TDI tar is very complicated and costly. One method for disposing of the TDI tar is incineration. However, in this method, the TDI tar melts and adheres to the floor or bottom of the incinerator at the time of incineration, and various difficulties occur in handling it.
The effective utilization of TDI tar also means the utilization of NCO groups contained in the TDI tar. As stated above, the TDI tar has a very high softening point, and remains solid at temperatures which are usually employed for chemical reactions. Therefore, within this temperature range, the reactivity of the NCO groups is poor, and the reaction of the NCO groups with other substances such as polypropylene glycol or other polyols virtually does not proceed. On the other hand, in the solid form, the TDI tar cannot even be used as a briquet, a coking material, a pitch coke material, or a binder pitch for electrodes, etc. as in the case with coal tar pitch, because the TDI tar contains NCO groups.
Since TDI tar is solid at room temperature and has a softening point of as high as about 200.degree. C, it has been impossible to liquefy the TDI tar, and this is the reason why the TDI tar has not been effectively used up to now. Merely heating the TDI tar only results in the forming of a viscous liquid of a viscosity which is not feasible to use and undesirable phenomena such as the evolution of decomposition gases occur. Furthermore, even when a solvent of the usually employed types is mixed with the TDI tar, the TDI tar is only swollen, or dissolved slightly in an amount of about several percent, and it has been completely impossible to obtain a uniform solution. For this reason, no practical utility for the TDI tar has been found.
It has now been found that TDI tar is dispersible or soluble in specific aromatic solvents to convert the TDI tar in a liquid form (hereinafter liquid TDI tar), and that by combining such a liquid TDI tar as a curing agent with a polyol or isocyanate-reactive tar as a base compound, a urethane composition of superior properties can be obtained at low cost.